Computer-to-plate (CTP) is an imaging technology used in printing processes, in which imaging data is output directly from a computer to a printing plate by exposing the printing plate to light energy according to the imaging data. In the case of an external drum imaging device, a printing plate or sleeve is positioned on an imaging cylinder of the imager. As the drum rotates around its longitudinal axis, an image head moves in the axial direction and focuses one or more laser beams modulated with imaging data onto the plate to image a pattern on the surface of the printing plate.
Although the description is in terms of imaging a plate on an external drum imaging device, the same principle applies to imaging a sleeve on the external drum imaging device, or to using a flat-bed imaging device.
It is known to image a photopolymer plate with a laser ablatable mask (“LAMS”) coating thereon, in which case the laser beam or beams ablate a pattern in the LAMS material to form a mask through which the plate can be exposed to ultraviolet and processed to produce a printing plate with the image pattern indirectly engraved thereon.
In direct engraving, the laser beam or beams ablate the image pattern directly on an elastomeric printing plate. The term “elastomer” and the adjective “elastomeric” are used herein broadly to include a rubber plate and a polymer plate. The terms are used interchangeably with the term “rubber.”
Direct engraving typically requires much higher laser energy than ablating a LAMS coating. While the typical thickness of a LAMS coating is in the order of 3 μm, direct engraving requires ablating plate material to produce a much deeper pattern, usually in the order of 500 μm deep. As a result, direct engraving of elastomeric plates requires about three orders of magnitude more laser power than does ablating a LAMS coating.
High power lasers, such as sealed-off carbon dioxide (CO2) lasers and fiber lasers can provide output power up to kilowatts (kW) range, e.g., 500 W to 3 kW. Coherent, Inc., of Santa Clara, Calif., offers off-the-shelf 500 W and 1000 W CO2 lasers. IPG Photonics Corporation, Oxford, Mass. offers off-the-shelf single-mode fiber lasers of up to 1000 W, and can make custom fiber lasers of up to 3 kW. Such lasers can engrave with satisfactory quality. However, imaging is relatively slow. Existing direct engraving systems have productivity around 0.5 square meters per hour (m2/h), whereas imaging of photopolymer plates can achieve as fast as 8 m2/h.
Direct engraving systems are known that use a CO2 or fiber laser and an optical-beam modulator such as an acousto-optic modulator (AOM) to switch the laser-beam power on the plate on and off. The switching speed of an AOM is limited. High power lasers with an AOM have a switching speed that is merely in the low kilohertz (kHz) range. To achieve a speed of 8 m2/h, however, requires a switching speed in a megahertz (MHz) range. In general, the higher the laser power, the slower the switching speed.
One expensive solution to the imaging speed limitation is to use several engraving systems in parallel. Such a solution however is expensive and requires a large space to operate the multiple engraving systems.
Beside the low productivity, direct engraving of an elastomeric plate can produce a large amount of evaporated elastomer. The vapour may disturb focusing of laser beams and form a plasma, which may cause melting of small details on the plate surface.